In most high performance communication systems (e.g., digital cell phone systems), carrier recovery is performed in the digital domain because the computing power is readily available. However, in other communications systems (e.g., optical or millimeter wave communications systems) performing carrier recovery in the digital domain may not be feasible because (for example) of the cost of an analog-to-digital converter (ADC) with requisite performance characteristics. As a result, these communications systems have resorted to using constant amplitude modulated communications (e.g., quadrature phase-shift keying or QPSK) along with Costas carrier recovery techniques, but it would be desirable to use more complex amplitude and phase modulated schemes (e.g., m-quadrature amplitude modulation or m-QAM) so as to increase channel throughput. Therefore, there is a need for an analog-domain carrier recovery technique for amplitude and phase modulated communications schemes.
Some examples of conventional systems are: U.S. Pat. Nos. 7,079,574; 8,081,948; U.S. Patent Pre-Grant Publ. No. 2010/0093299; U.S. Patent Pre-Grant Publ. No. 2011/0075713; U.S. Patent Pre-Grant Publ. No. 2011/0122932; U.S. Patent Pre-Grant Publ. No. 2012/0033965; PCT Publ. No. WO2012074551.